ARM: vexpress: Don't select VEXPRESS_CONFIG

CONFIG_VEXPRESS_CONFIG has 'default y if ARCH_VEXPRESS', so selecting is
unnecessary. Selecting it also prevents setting CONFIG_VEXPRESS_CONFIG
to a module.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>